This invention relates to nanotechnology which is the science of controlling and manipulating particles (atoms and molecules) smaller than 20 nanometers. A nanometer is approximately 75 thousand times smaller than the width of human hair, or about 3 to 8 atoms wide. Specifically this invention relates to the production and applications of nanoparticles that behave like small magnets, i.e. nanomagnets
Since the discovery of doped nanocrystals (DNC) in 1994 made from ZnS with Mn2+ as the dopant, (See R. N. Bhargava et. al. Physical Rev. letters 72, 416 (1994)). Several applications of these class of DNC's have appeared (See U.S. Pat. Nos. 5,422,489, 5,422,907, 5,446,286, 5,455,489 and 5,637,258). In all the earlier work on doped nanocrystals, the size of the host was estimated to be less than 10 nm for efficient generation of light. These materials were developed over several years for various applications. In all the applications and products, the light generated in the nanocrystals was associated with the dopant (also called an impurity or activator) while the absorption of the exciting light was associated with the host.
An active dopant when incorporated in a semiconductor provides an additional free electron or hole that modulates the electrical conductivity of the semiconductor and thereby provides the precise control of device parameters. The role of these dopant is well documented in semiconductors. On the other hand, if a dopant, usually referred as an activator, is incorporated in an insulator, it can act as a very good radiative recombination center for the excited electrons and provide efficient light. These materials are referred as phosphors. The difference between semiconductor dopant and phosphor activator being that activator does not modulate the electrical conductivity while dopant in semiconductor does.
On the other hand if a dopant or impurity is incorporated in a semiconductor or insulator nanocrystal of 2 to 5 nm size, the luminescent properties of the dopant are strongly affected by the quantum confinement provided by the dielectric boundary of the nanocrystal. The dopant in a nanocrystal, henceforth referred as quantum confined atom (QCA), generates high luminous efficiency. This observed high efficiency along with other experiments performed on individual nanocrystal have confirmed unequivocally that only a single dopant per nanocrystal was incorporated (c.f. (M. D. Barnes et. al, J. Phys. Chem. B 104 6099, 2000; and A. P. Bartko et. al. Chemical Physics Letters 358 459, 2002.) It is also known that two or more such dopants/activators per nanocrystal will lead to phenomena of concentration quenching and drastically reduce the quantum efficiency. It is important to note, that the properties of the single atomic ion activator in nanocrystal of size less than 10 nm are modified due to quantum confinement. This has been published in detail (R. Bhargava et. al. Phys. Rev. Letts. 72, 416, 1994; R. Bhargava et. al. Phys. Stat. Sol., (b) 210, 621, 1998). Thus, for luminescent applications of QCA-based nanocrystals, we can conclude that only the luminescent properties of the QCA activator has been modified without any changes in the optical properties of the nanocrystalline host.
In the case of QCA based nanophosphors, recent studies (M. D. Barnes et. al, J. Phys. Chem. B 104 6099, 2000; and A. P. Bartko et. al. Chemical Physics Letters 358 459, 2002) suggest strongly that there is either one activator-ion or zero activator-ion per nanocrystal i.e. creating digital doping. The probability of incorporation of the single activator-ion in a nanocrystal critically depends on the preparative methods, the starting concentration of activator-ion with respect to the ion it replaces and the size of the host. To incorporate a single dopant ion in a nanocrystal, the chemistry of preparation has to be adjusted. For example, the probability of incorporating the dopant-ion in the host decreases rapidly as the size of the host size decreases. In order to increase the probability of incorporation in smaller particles, we increase the concentration of the dopant-ion in starting reactant five to ten fold. This higher concentration of dopants in starting solution ensures that the smallest of the particles have a single dopant ion that is necessary for the light generation.
Recently we have demonstrated that in QCA based nanomaterials, the efficiency of the light emanating from a single caged atom (ion) is the highest when the particle size is less than 5 nm. As the size decreases from 10 nm to 2 nm, the light from the caged atom increases non-linearly as the size decreases. Recent developments in the preparation and separation of the particles, along with microscopic-optical studies of individual nanophosphors had led to a greater degree of understanding of the role of a single atom in a nanoparticle. Several devices and applications and products now emerge from this newly found science of QCA for the next generation devices using nanotechnology.
QCA Based Nanomagnets
A single atom of a dopant (activator) is confined in a cage of a 2 nm to 5 nm size nanoparticle of the host compound (8 to 20 atoms in a linear chain) is schematically represented in FIG. 1A where the atom is represented as a charged cloud, a correct quantum mechanical representation of an atom. Upon further decrease of size of the host-cage as depicted in FIG. 1B, the QCA due to quantum confinement shows extraordinary changes in the charge distribution and influences both the optical and magnetic properties.
Recent research has established for the first time that a single atom in the cage experiences the ‘quantum confinement’ effect and that generates efficient light. This discovery demonstrates that the properties of a single atom can be manipulated controllably, and will impact optical and magnetic devices and is expected to become a formidable branch of Nanotechnology. Furthermore, the QCA's produced herein show self aligning (self-organizing) properties which can lead to self assembling nanodevices which is a significant step as it moves nanoparticles from the laboratory to commercially useful devices.
The present application is directed to the preparation and use of a class of nanoparticles called Quantum Confined Atoms or QCA based nanocrystals. A QCA based nanocrystal is a particle of material comprising a plurality of host atoms in a size less than 10 nm with a single atom of a QCA-dopant confined within. The QCA's have unique luminescent and optical properties that include efficient linearly-polarized light as depicted in FIG. 2. FIG. 2 shows the emission pattern from a single Eu3+ ion (left) and the pattern generated by the same particle after introducing a sheet polarizer at 45° (right). The pattern on the right clearly indicates linearly polarized emission from a single ion (the dopant) in a nanocrystal cage. The linearly-polarized light observed in luminescence (c.f. M. D. Barnes et. al, J. Phys. Chem. B 104 6099, 2000; and A. P. Bartko et. al. Chemical Physics Letters 358 459, 2002) is due to a fixed polarization of the electric vector associated with a single dopant ion. Electromagnetic coupling utilizing Maxwell's equation suggest that the magnetic-field vector associated with the QCA must also be polarized. Thus if the host-cage and dopant-ion are chosen to have the strong electron magnetic-spin, the polarized magnetic field generated by the QCA will transform the nanocrystal to a nanomagnet.
In case of nanomagnets, the interaction associated with a QCA goes beyond what we have observed in QCA based luminescent materials. The doped QCA which is chosen to have large magnetic moment due to unpaired electron-spins, will become spin-polarized due to the quantum confinement imposed by the nanosize host. This is shown in FIG. 3A, with host spins randomly oriented. Since the chosen magnetic QCA-ion is now spin-polarized, it will impose a large magnetic field to its neighboring atoms of the nanocrystalline host. If the host atoms have also magnetic moment due to unpaired spins, all such spins will align due to strong magnetic-interaction referred as spin-spin exchange interaction. Due to this strong mutual magnetic interaction among the QCA dopant and the host atoms, all the spins align as depicted in FIG. 3B and make the entire nanocrystal a nanomagnet as shown in FIG. 3B. As discussed above in case of the QCA based luminescent materials the modified luminescent properties of the QCA do not modify the luminescent properties of nanocrystal host. Thus nanomagnets have additional modification imposed upon the QCA based nanomaterials due to direct spin-exchange interaction. This is fundamental breakthrough and distinguishes this patent from earlier QCA or doped nanocrystal patents issued to Nanocrystals Technology.